This invention relates to a method and a device for demodulating a received signal including a pilot signal, and more particularly to, a method and a device for demodulating a received signal including a pilot signal while suppressing the bit error rate (BER) based on a unique word as a pilot signal in mobile satellite communications that incur fast fading on the transmission path.
FIG. 1 shows a mobile terminal for mobile satellite communications. This mobile terminal is, for example, a portable telephone, which comprises an antenna 91, a signal converter 92, a demodulation circuit 93, a voice signal generator 94 and a speaker 95. The antenna 91 receives, through a satellite, a transmit signal as a continuous signal that has a unique word as a pilot signal in multiple sections. The signal converter 92 converts an analogue received signal into a digital signal. The demodulation circuit 93 demodulates a digital signal converted by the signal converter 92. The voice signal generator 94 converts digital demodulation signal from the demodulation signal 93 into an analogue voice signal. The speaker 95 performs the electricalxe2x80x94acoustical conversion of output of the voice signal generator 94, generating voice.
In the composition above, an analogue receive signal received by the antenna 91 is converted into a digital signal. The converted digital signal is demodulated by the demodulation circuit 93, and then the demodulated signal is converted into an analogue voice signal by the voice signal generator 94. The analogue voice signal converted by the voice signal generator 94 is output as voice from the speaker 95.
In the satellite communication system, for the purpose of reducing the consumed power and miniaturizing the antenna as well as retaining a low BER even under a low ratio of carrier power to noise power (Eb/NO), an error correction encoding with a large encoding gain, or a high-efficiency encoding technique is used.
FIG. 2 shows the composition of the demodulation circuit 93 in FIG. 1. The demodulation circuit 93 comprises a delay circuit 101 that receives the receive signal as an input, a data demodulator 102 that is connected to the delay circuit 101 and demodulates data, a unique word demodulator 103 that demodulates a unique word included in a received signal and a data distortion estimator 104 that includes a Wiener filter and that estimates the fading distortion of data in the received signal based on the fading distortion of the unique word. The demodulation circuit 93 is described in Meyr, xe2x80x9cDigital Communication Receiverxe2x80x9d, John Wiley and Sons, pp.744-747, 1997. This circuit demodulates a received signal compensating a distortion (hereinafter referred to simply as xe2x80x98fading distortionxe2x80x99) of the received signal based on a flat fading in multi-path fading that includes a frequency selective fading with a frequency characteristic and a flat fading with no frequency characteristic.
FIG. 3 shows a format burst type transmit/receive signal different from the continuous signal communicated by the mobile terminal in FIG. 1. For example, one burst comprises multiple data (though data compose data sequence, herein referred to simply as xe2x80x98dataxe2x80x99) D1 to D5 with multiple symbols and multiple unique words (though unique words compose unique-word sequence, herein referred to simply as xe2x80x98unique word(s)xe2x80x99) UWm (m=1 to 4) with multiple symbols, and is composed of 100 to 150 symbols as a whole. At four boundaries between data D1 and D5, unique words UWm as pilot signals are inserted. The unique word UWm has multiple symbols (in some cases, a single symbol) according to BER targeted, and the value of phase modulation of each symbol is known in the mobile terminal. The length (number of symbols) of unique words UW1 to UW4 is set so that it becomes minimum in the range that a certain error rate can be kept. This format signal is communicated using TDMA (time division multiple access). For example, each symbol is determined by phase-modulating (BPSK: binary phase shift keying) carrier wave with a phase of 0 and xcfx80 according to binary data of 1 and xe2x88x921.
In FIG. 2, a continuous-wave received signal with unique words UWm as pilot signals inserted at given intervals is input to an input terminal IN, then supplied to the delay circuit 101 and the unique word demodulator 103. Unique words UWm in the received signal are demodulated by the unique word demodulator 104, then input to the data distortion estimator 104. The unique word demodulator 104 estimates the fading distortion of each symbol of data D1 to D5 in the received signal based on the fading distortion of unique word UWm calculated from a known value of the unique word (for example, 0 by phase-demodulating +1, xcfx80 by phase-demodulating xe2x88x921) and a value of demodulated unique word UWm, then outputting it to the data demodulator 102. The data demodulator 102 demodulates data while compensating data in the received signal delayed for a given time by the delay circuit 101 using the estimation amount of fading distortion, and then the demodulated signal is output from an output terminal OUT. Thus, data can be demodulated compensating the fading distortion of data transmitted as a continuous wave.
FIG. 4 shows another demodulation circuit that can be used in place of the demodulation circuit 93 in FIG. 2. This circuit is described in S. Sanpei, xe2x80x9cCompensation System of Fading Distortion of 16QAM for Overland Mobile Communicationsxe2x80x9d, Technical Report of IEICE B-11, Vol.J72-B-11, No.1, pp.7-15, 1989. It demodulates compensating the fading distortion of a continuous-wave received signal
ĉ(kxe2x88x921), ĉ(k) and ĉ(k+1)
that one unique word symbol is inserted to every (Nxe2x88x921) information symbols. In FIG. 4, when a received signal is input to an input terminal IN, fading distortion estimators 111, 112 and 113 calculate the estimation values: of (kxe2x88x921 th, kth and (k+1 th unique words in the received signal delayed sequentially. These estimation values are multiplied by a zero-order or first-order interpolation coefficient:
Q1, Q0, or Q.1
at multipliers 114, 115 and 116, then added by an adder 117. The adder 117 outputs, as the result of addition, c {k+(m/N)} that is the fading distortion of mth information symbol in kth information symbol sequence. The fading distortion of information symbol is brought into 1/c{k+(m/N)} by a reciprocal transformer 118, then output to a multiplier 120. The multiplier 120 multiplies information symbol in a received signal delayed by a delay circuit 119 by 1/c {k+(m/N)} output from the reciprocal transformer 118, thereby demodulating data, which is output from an output terminal OUT. Thus, the received signal can be demodulated compensating the envelope curve and phase of the received signal distorted with the fading distortion.
The demodulation device in FIG. 2 can demodulate a continuous-wave received signal that unique words are inserted into continuous data at given intervals, at a required BER. However, when it receives a burst signal with a frame format that unique words are inserted into several positions of a 100 to 150 symbol data sequence, since the fading distortion of a data sequence located at both ends is estimated using the fading distortion of the unique word only at one side, the estimation precision of the fading distortion of data at both ends reduces. Therefore, under the condition of fast fading or low Eb/NO ratio, a required BER cannot be obtained. For example, when Eb/NO=2 under the conditions of signal-to-fading intensity C/M=7 dB and Doppler frequency=0.01, a required BER corresponds to a deterioration of 0.5 dB from the theoretical value. Therefore, when the Wiener filter is replaced by a Kalman filter etc. with a higher estimation precision, the amount of operation increases since a matrix-like manner requiring a large amount of operation is necessary.
Also, in the demodulation device in FIG. 4, when using the zero-order interpolation coefficient, one value of a unique word is kept over the corresponding (Nxe2x88x921) information symbols, and when using the first-order interpolation coefficient, the fading distortion of information symbol is estimated by connecting between adjacent unique words with a straight line. Therefore, under the condition of fast fading or low Eb/NO ratio, a required BER cannot be obtained.
In order to enhance the estimation precision of fading distortion of data, it is, needless to say, necessary to increase the detection precision of fading distortion of a unique word. However, when the number of symbols for unique a word is increased for that purpose, the ratio of symbol number for data decreases relatively and therefore the efficiency of data transmission reduces.
Accordingly, it is an object of the invention to provide a method and a device for demodulating a received signal including a pilot signal that even when fast fading occurs on the transmission path, the fading distortion of data can be estimated at a high precision and a low BER can be obtained.
It is a further object of the invention to provide a method and a device for demodulating a received signal including a pilot signal that a low BER can be obtained compensating fast fading without reducing the ratio of symbol number for data in burst signal.
According to the invention, a method for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises the steps of:
(1) setting part of data as an extended unique word;
(2) detecting the fading distortion of the unique word as the pilot signal as a first fading distortion and detecting the fading distortion of the extended unique word as a second fading distortion;
(3) estimating the fading distortion of data based on the first and second fading distortions; and
(4) demodulating data based on the fading distortion of data.
According to another aspect of the invention, a method for demodulating a received signal including pilot signal while estimating the fading distortion of data by detecting the fading distortion of unique word as pilot signal inserted into multiple sections of a received signal, comprises the steps of:
(5) setting part of data adjacent to the unique word as a first extended unique word attached to the unique word;
(6) setting part of data that is apart from the unique word as a second extended unique word independent of the unique word;
(7) estimating the fading distortion of the first and second extended unique words;
(8) estimating the fading distortion of data in the received signal based on the fading distortion of the first and second extended unique words; and
(9) demodulating data in the received signal based on the estimation result of fading distortion of data in the received signal.
According to another aspect of the invention, a method for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises the steps of:
(10) detecting the first-stage fading distortion of the unique word in the multiple sections by comparing the unique word in the multiple sections with a known unique word;
(11) estimating the first-stage fading distortion of data in the received signal by conducting the first-stage MAP operation based on the first-stage fading distortion of unique word in the multiple sections;
(12) demodulating the received signal based on the first-stage fading distortion of data in the received signal and then outputting demodulated signal;
(13) comparing the demodulated signal with a predetermined threshold value and then outputting a binary signal as the comparison result;
(14) detecting the second-stage fading distortion of the unique word in the multiple sections by comparing the unique word in the multiple sections with a known unique word;
(15) detecting the fading distortion of part of data adjacent to the unique word in the multiple sections and part of data apart from the unique word in the multiple sections by comparing the part of data adjacent to and apart from the unique word in the multiple sections with the binary signal;
(16) estimating the second-stage fading distortion of data in the received signal by conducting the second-stage MAP operation based on the second-stage fading distortion of unique word in the multiple sections and the fading distortion of the part of data adjacent to and apart from the unique word in the multiple sections; and
(17) demodulating data in the received signal delayed for a predetermined time based on the second-stage fading distortion of data.
According to another aspect of the invention, a method for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises the steps of:
(18) detecting the first-stage fading distortion of the unique word in the multiple sections by comparing the unique word in the multiple sections with a known unique word;
(19) calculating the first-stage spline interpolation curve based on the first-stage fading distortion of the unique word in the multiple sections and then estimating the first-stage fading distortion of data in the received signal from the first-stage spline interpolation curve;
(20) demodulating the received signal delayed for a predetermined time based on the first-stage fading distortion of data in the received signal and then outputting the demodulated signal;
(21) comparing the demodulated signal with a predetermined threshold value and then outputting a binary signal as the comparison result;
(22) detecting the second-stage fading distortion of the unique word in the multiple sections by comparing the unique word in the multiple sections of the a received signal delayed for a predetermined time with a known unique word;
(23) detecting the fading distortion of part of data adjacent to the unique word in the multiple sections and part of data apart from the unique word in the multiple sections by comparing the part of data adjacent to and apart from the unique word in the multiple sections with the binary signal;
(24) calculating the second-stage spline interpolation curve based on the second-stage fading distortion of the unique word in the multiple sections and the fading distortion of the part of data adjacent to and apart from the unique word in the multiple sections, and then estimating the second-stage first fading distortion of data in the received signal delayed for a predetermined time from the second-stage spline interpolation curve;
(25) inputting the second-stage fading distortion of the unique word in the multiple sections and the fading distortion of the part of data adjacent to and apart from the unique word in the multiple sections to a Wiener filter, thereby estimating the second-stage second fading distortion of data in the received signal delayed for a predetermined time; and
(26) demodulating data in the received signal delayed for a predetermined time based on the second-stage first and second fading distortions of data.
According to another aspect of the invention, a method for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises the steps of:
(27) detecting the fading distortion of the unique word in the multiple sections by comparing the unique word in the multiple sections with a known unique word;
(28) calculating a MAP algorithm based on the fading distortion of the unique word in the multiple sections;
(29) estimating the fading distortion of data in the received signal from the calculation result of the MAP algorithm; and
(30) demodulating data in the received signal based on the fading distortion of data.
According to another aspect of the invention, a method for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of the unique word as the pilot signal inserted into multiple sections of the received signal, comprises the steps of:
(31) detecting the fading distortion of the unique word in the multiple sections as first fading distortion by comparing the unique word in the multiple sections of the received signal with a known unique word;
(32) calculating a first MAP algorithm based on the first fading distortion;
(33) estimating the fading distortion of data in the received signal as second fading distortion from the calculation result of the first MAP algorithm;
(34) demodulating data based on the second fading distortion;
(35) conducting the provisional hard decision of the demodulated data and then outputting a value of the provisional hard decision;
(36) detecting the fading distortion of the unique word in the multiple sections as third fading distortion by comparing the unique word in the multiple sections of the received signal with a known unique word;
(37) detecting the fading distortion of data in the received signal delayed for a predetermined time as fourth fading distortion by comparing the data with the value of the provisional hard decision;
(38) calculating a second MAP algorithm based on the third and fourth fading distortions;
(39) estimating the fading distortion of data in the received signal as fifth fading distortion from the calculation result of the second MAP algorithm; and
(40) demodulating the data in the received signal delayed for a predetermined time based on the fifth fading distortion.
According to another aspect of the invention, a method for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises the steps of:
(41) estimating the first fading distortion of data based on the fading distortion of the unique word;
(42) demodulating the data based on the first fading distortion of data;
(43) detecting the fading distortion of an extended unique word from the fading distortion of the unique word and the fading distortion of the data obtained based on the demodulation result of the data;
(44) estimating the second fading distortion of the data based on the fading distortion of extended unique word;
(45) demodulating the data based on the second fading distortion of the data; and
(46) discontinuing the steps (43) to (45) according to the level of the first and second fading distortion of the data, then outputting the modulation result of the data at the step (42)
According to another aspect of the invention, a device for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted in to multiple sections of the received signal, comprises:
means for detecting the fading distortion of the unique word as first fading distortion and detecting the fading distortion of extended unique word set at part of the data as second fading distortion;
means for estimating the fading distortion of the data in the received signal based on the first and second fading distortions; and
means for demodulating the data in the received signal based on the fading distortion of the data.
According to another aspect of the invention, a device for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises:
means for detecting the fading distortion of the unique word in the multiple sections of the received signal by comparing a unique word in the multiple sections and a known unique word;
means for calculating a MAP algorithm based on the fading distortion of the unique word in the multiple sections, and estimating the fading distortion of data in the received signal from the calculation result; and
means for demodulating data in the received signal based on the fading distortion of the data.
According to another aspect of the invention, a device for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises:
a first detection means for detecting the fading distortion of the unique word in the multiple sections of the received signal as first fading distortion by comparing the unique word in the multiple sections and a known unique word;
a first calculation means for calculating a first MAP algorithm based on the first fading distortion;
a first estimation means for estimating the fading distortion of data in the received signal as second fading distortion from the calculation result of the first MAP algorithm;
a first demodulation means for demodulating data delayed for a predetermined time based on the second fading distortion;
a decision means for conducting the provisional hard decision of the demodulated data and then outputting a value of the provisional hard decision;
a second detection means for detecting the fading distortion of the unique word in the multiple sections of the received signal delayed for a predetermined time as third fading distortion by comparing the unique word in the multiple sections and a known unique word;
a second calculation means for calculating a second MAP algorithm based on the third and fourth fading distortions;
a second estimation means for estimating the fading distortion of data in the received signal as fifth fading distortion from the calculation result of the second MAP algorithm; and
a second demodulation means for demodulating data delayed for a predetermined time based on the fifth fading distortion.
According to another aspect of the invention, a device for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted in to multiple sections of the received signal, comprises:
a first detection means for detecting the first-stage fading distortion of the unique word in the multiple sections of the received signal as first fading distortion by comparing a unique word in the multiple sections and a known unique word;
a first spline interpolation estimation means for calculating a first-stage spline interpolation curve based on the first-stage fading distortion of the unique word in the multiple sections, and estimating the first-stage fading distortion of data in the received signal from the first-stage spline interpolation curve;
a first demodulation means for demodulating the a received signal based on the first-stage fading distortion of data in the received signal and then outputting demodulated signal;
a decision means for comparing the demodulated signal with a predetermined threshold value and then outputting a binary signal;
a second detection means for detecting the second-stage fading distortion of the unique word in the multiple sections of the received signal by comparing a unique word in the multiple sections and a known unique word, and detecting the fading distortion of part of data adjacent to the unique word in the multiple sections and part of data apart from the unique word in the multiple sections by comparing part of data adjacent to and apart from the unique word with the binary data;
a second spline interpolation estimation means for calculating a second spline interpolation curve based on the output of the second detection means, and estimating the second-stage first fading distortion of data in the received signal from the second spline interpolation curve;
a Wiener filter means for being fed with the output of the second detection means and then estimating the second-stage second fading distortion of data in the received signal; and
a second demodulation means for demodulating data in the received signal based on the second-stage first and second fading distortions of the data.
According to another aspect of the invention, a device for demodulating a received signal including a pilot signal while estimating the fading distortion of data by detecting the fading distortion of a unique word as the pilot signal inserted into multiple sections of the received signal, comprises:
a first estimation means for estimating the first fading distortion of data based on the fading distortion of the unique word;
a first demodulation means for outputting first demodulated signal of the data based on the first fading distortion of data;
a detection means for detecting the fading distortion of an extended unique word from the fading distortion of the unique word and the second fading distortion of the data estimated based on a value of the provisional hard decision of the demodulated signal of the data;
a second estimation means for estimating the second fading distortion of the data based on the fading distortion of the extended unique word;
a second demodulation means for outputting second demodulated signal of the data based on the second fading distortion of the data;
an output circuit for outputting the demodulated signal to a subsequent-stage circuit; and
a control means for selectively connecting the first or second demodulation means to the output circuit according to the level of the first and second fading distortion of the data, for controlling the detection means, the second estimation means and second demodulation means not to operate when the first demodulation means is connected to the output circuit.
In the method and device for demodulating a received signal including a pilot signal of this invention, a transmit signal, e.g., a burst signal, that unique words as pilot signals are inserted into multiple sections is A/D-converted, and then the fading distortion of unique words in the multiple sections is detected by comparing the unique words in the received signal and known unique words. Then, based on the fading distortion of the unique words, a predetermined calculation is conducted, thereby the fading distortion of data in the received signal is estimated. The received signal is then demodulated based on this estimation value. The received signal demodulated is compared with a threshold value, e.g., zero, and is subject to the provisional hard decision to output binary signal of 1 or xe2x88x921. The unique words of the received signal are compared again with known unique words, thereby the fading distortion of unique words is detected.
Simultaneously, data adjacent to the unique words and data apart from the unique words are compared with the binary signal obtained in the provisional hard decision, thereby the fading distortion of extended unique words is detected. Then, the fading distortion of unique words and the fading distortion of data adjacent to and apart from the unique words are averaged between multiple symbols. The average of fading distortion thus obtained is input to the calculation means. The calculation means estimates the fading distortion of data according to the fading distortion of the unique words. Using this fading distortion of data, data of the received signal is modulated.